Since there are enemy armies in range to threaten the Egyptian capital I would fully expect the AI to be attempting to send their troops to Sarkel. Defense is a dirty word.

I know nothing of code, but it astonishes me that even the most basic tenets of strategy are entirely absent from MTWII's AI. I remember someone said that reloading the save (Inevitable in our style of game) breaks some aspects of the AI, but in my experience it's useless whether you do that or not. More than once in my Total Independence campaign a large AI army would walk past one of their former cities that I had recently conquered and left entirely undefended trying to reach one of their remaining cities that my own forces were closer to or already held under siege.

In other words, I, for one, am very glad we have a competent opponent for this crusade.